The principle of the heat pump was initially proposed by Nicholas Carnot in 1824. Thirty years later, Lord Kelvin suggested refrigerating equipment could be used for heating. Although several manufacturers built heat pumps in the 1930s, it was not until the 1950s that heat pumps began to be mass produced. Conventionally, the heat was exchanged with the ambient outside air, and refrigerants, e.g., hydrocarbons or fluorocarbons, were used in the process. The use of refrigerants, such as hydrocarbons or, proved to have a negative environmental impact. Today, numerous types of heating and cooling systems are used for controlling the temperature of various thermal loads, and efforts are made to minimize pollutants in the environment. Many existing heating and cooling systems, including heat pumps, air conditioners, refrigeration units, and the like, operate on the same thermodynamic principles and utilize the same basic components.
Most commonly, these basic components include a compressor, an expander, a load heat exchanger, and an external heat exchanger. Each of these components is connected with a piping system which carries a circulating fluid, e.g., a refrigerant (liquid or gas) or water (or an antifreeze or alcohol water solution) throughout the system. In order for this type of system to operate, heat must be exchanged with the environment. This heat exchange with the environment may be accomplished by directing the circulating fluid to an outdoor coil, e.g., an external heat exchanger, where thermal energy is exchanged between the water/refrigerant contained in the coils and the outside air.
Using outside air as the sink or source for a heat exchange process is problematic due to the variability of air temperature. A heat pump operating during the winter requires the external heat exchanger to absorb thermal energy from the outside air. The heating system, however, loses its efficacy and efficiency as the outside temperature falls because less thermal energy can be extracted from the outside air, and therefore, these heating systems must have a secondary heat back up, e.g., electric heating strips. Similarly, a cooling system, such as an air conditioner, encounters the same efficacy and efficiency problems when the outside temperature rises.
A ground source heat pump works like other heat pumps in that it has a basic refrigerating circuit but, instead of extracting energy from the air, it uses the heat stored in the ground. Ground source heat exchange is a potentially more efficient and effective way to perform the external heat exchange required by many heating and cooling systems. Unlike air temperatures, the ground temperature is a relatively constant temperature that ranges from about 44° F. to about 76° F. at a depth below the local frost line. Additionally, the ground can act as a virtually limitless energy source or heat sink.
Currently, plastic pipe is typically used with ground source heat pumps, and is buried in the earth, or disposed in lakes, rivers, ponds, water wells, and the like. Copper coils containing refrigerants, and flat steel plates disposed in rivers, lakes, and ponds, are also used. In using plastic tubing, or pipes, one method is to install the pipes horizontally with a trencher or back hoe. This requires 350′ per ton of heat pump, e.g., a five ton heat pump needs 1750′ of pipe and trench. Examples contained herein regarding pipe length and pipe and trench length figures throughout are for the average annual ambient temperatures of Charleston W. Va. These figures will vary for other locations based upon average annual ambient temperature. In general, two basic types of connecting horizontal loop arrangements are utilized, which include connecting a closed loop in series so that only one long loop is present and in parallel so that several loops are disposed to use the same input/output pipes. Vertical loops are also installed by a drilling machine in either a parallel or series configuration, which requires a 150′ to 225′ of borehole per ton of heat pump which requires a pipe length of 300′ to 450′ per ton of heat pump.
Although, ground source heat pump heating and cooling systems generally include many of the same essential components as other heat pump heating and cooling systems, except that the external heat exchanger operates in a different manner. The external heal exchange process of a ground source heat pump heating or cooling system is generally accomplished by one of two methods. One method is simply to extend the refrigerant fluid carrying coil into the soil, thereby directly exchanging heat with the ground, e.g., a direct expansion (DX) refrigerant loop. The second method utilizes a circulating heat exchange fluid, e.g., water or other aqueous solution, to carry thermal energy between the ground and the refrigerant heat exchanger and on the the thermal load. Typically, this circulating heat exchange fluid travels in a piping system between a subterranean heat exchanger, where heat is exchanged with the ground, and the refrigerant heat exchanger and on to the thermal load, where heat is exchanged with the heating or cooling system. When the water/refrigerant carrying coil of the heating or cooling system contacts this circulating heat exchange fluid, heat is exchanged directly with the circulating heat exchange fluid and, thereby, indirectly with the ground.
Most existing ground source heating and cooling systems use a circulating heat exchange fluid to transfer heat between the system and the ground. Heretofore, geothermal systems of this type typically employ small size polyethylene pipe(s), and a dedicated loop field to service each individual thermal load. Most of these heat exchange loops are oriented vertically extending down into the earth. This limits the contractors who can install these systems and creates a muddy water mess at the ground surface which is unacceptable at many locations potentially increasing the cost of the installation. Where horizontal loops are used, they tend to require a large surface area. The heat exchangers with small sized polyethylene pipe loops that are oriented horizontally are typically buried four or more feet beneath the ground surface, and take up a great deal of surface area. Many locations are inadequate in surface area size to accommodate these horizontal loops.
Direct exchange geothermal heat pumps use a single loop circulating refrigerant through tubes that are in direct contact with the ground. The refrigerant circulates through a loop of copper tube buried underground, and exchanges heat with the ground. Water-source, and water loop, heat pumps are considered different because they use water or a water antifreeze mixture. Most such systems have two loops, including a primary refrigerant loop that is contained in the heat pump cabinet where it exchanges heat with a secondary water loop that is buried. The secondary loop is typically made of high-density polyethylene pipe containing a mixture of water and anti-freeze, such as propylene glycol, monopropylene glycol, denatured alcohol, methanol, or the like. After leaving the internal heat exchanger, the water flows through the secondary loop outside the building to exchange heat with the ground before returning. The secondary loop is placed below the frost line, or submerged in a body of water, or well, if available. Ground moisture aids in the heat exchange, and therefore, where the ground is naturally dry, sprinkler (or soaker) hoses may be buried with the ground loop to keep it wet.
Efforts to devise modular geothermal heat exchangers have been made in the past, but these devices tend to be too large for small homebuilders, do-it-yourself homeowners, for temporary or seasonal habitats, or the like because these devices typically require deep/long trenches or large bodies of water in which to place the heat exchanger. U.S. Pat. No. 5,224,357 teaches a ground source heat exchanger having modular tube bundles adapted to be placed within narrow excavation in the group that also utilizes thermal conductive materials such as metals, and more specifically, copper or aluminum; however, that device is quite different from the present invention having many more coils of tubing in each modular bundle, and the bundles requiring much deeper trenches. A source of water, such as a soaker hose, is disclosed to maintain proper moisture levels directly above the modular bundles.
Similarly, U.S. Pat. No. 5,339,890 teaches a modular ground source heat pump system with subterranean piping installation constructed of a plurality of modular heat exchange units, which utilizes a tube within an insulated tube structure that requires deep holes, long trenches, wells, or bodies of water. Another patent along the same design is U.S. Pat. No. 5,533,355 which teaches a ground source heat pump system wherein modular heat exchange units are utilized. These systems are limited to using a parallel connection system to the inlet/outlet piping instead of having the option of the modular units generating a single coil with the separate modules operating in series.
U.S. Pat. No. 5,651,265 teaches a more conventional ground source heat pump system with an internal heat exchanger and an arrangement of check valves to permit a single direction of refrigerant flow in both the heating and cooling modes. The system charge is the same for heating and cooling and the ground coil consists of a plurality of three pipe units—one pipe for inflow and two for out flow. US Patent Application No. 2010/0258266 teaches a modular system with dual loops, an inner loop disposed within a contained cylinder.
Each of these inventions require a great deal of space, a large body of water, or at least one very deep borehole, and as such, are difficult for your average consumer to utilize. As such, conventional ground source heat pumps tend to be used by businesses or property owners with lots of resources and large lots, because of the extra space required, and the high cost of installing conventional ground source heat exchangers.